The present invention relates to an injection moulding machine for injection moulding of plastic miniature parts.
Such injection moulding machines are generally known and basically comprise an injection aggregate and a closure arrangement for the form tools, whereby the injection aggregate has an extruder for plasticizing the synthetic material and a collection chamber for the plasticized mass, which is either ejected by an axial movement of the extrusion screw or by a separate injection ram out of the injection nozzle of the extruder. The form tools with their cavities are clamped between two clamping plates of the closure unit, whereby the injection nozzle of the extruder, during ejection of the plasticized mass, tightly contacts the die plate of the injection moulding machine by pressure of force.
In these machines, the melted material is pressed from the collection chamber of the extruder over feed or sprue channels arranged in a hot channel distributor into the cavities and, as a rule, molten material for a number of injection cycles is in the feed path between the collection chamber and the cavity. This material is subjected to repeated, abrupt compressions and accompanying temperature fluctuations over a period of time, and therefore, by the time it is filled into the cavities, the material already exhibits a reduction in quality or even particles of decomposition products. It is understood that products made with such material may exhibit faults or early fatigue symptoms.
A further and to date unsolved problem in the production of small parts is to be seen in the large quantity of sprue pieces occurring. Conventionally produced small parts always have a sprue piece, the weight of which can be well in excess of that of the part being produced, and which must be removed in a separate work step.
In this way, production with needle-free nozzle-closure devices can result in approx. 95% of the injected material being waste material and usable parts being produced from a mere 5% of the injected material. Even if this waste material is recycled into the plasticizing process, the characteristic properties of the respective plastics suffer and the miniature parts produced with this material no longer are able to completely meet the requirements expected from them.
In order to reduce the problem associated with hot channel distributors, DE-A-42""39""776 suggests providing an injection moulding machine for the simultaneous production of two miniature plastic parts, in which the injection arrangement is provided with two injection aggregates. In such a device, the plasticizing, the injection process and the post-pressure control are individually carried out for each miniature part to be produced, in order to reach the quality required for these parts with minimal production losses. In the production of high quality miniature parts it has proven particularly difficult to manufacture a number of forms simultaneously and within the required low permissible deviation, i.e. without intrinsic tension or production error and with precise filling weight.
It is therefore the object of the present invention to provide an injection moulding machine for producing qualitatively impeccable, wear resistant and durable parts, preferably miniature parts made of technical plastics, such parts being produced with as little waste as possible.
An arrangement described in the publication xe2x80x9cPlastiques Modernes et Elastomxc3xa8resxe2x80x9d, Vol. 35, Nr. 1, January/February 1983, Paris, France, Page 41, has a distribution block, whichxe2x80x94in contrast to conventional injection moulding machinesxe2x80x94is fixedly connected to the machine part. This distribution block bears on its front side, i.e. towards the form dies of the injection die a plurality of fixedly mounted injection nozzles which protrude into the first form plate of the injection moulding die. When filling the form cavities, these nozzles, which are fixedly connected to the distribution block and are hot, are brought into contact with the closed and cold injection moulding die via a plurality of infeed channels. After filling the form cavities, the cold die is separated from the hot distribution block and its nozzles. In this way the individual nozzles can again be brought up to the required high temperature, whilst the injection moulded parts can remain in the closed die in order to continue cooling, prior to opening the die for removing the injection moulded parts. The collection chamber of this assembly is located in the machine part, which has proven to be disadvantageous for the production of miniature parts because of the length of the injection channels. At this point it should be stressed that when producing miniature parts, a long retention time of the melt in the hot infeed channels leads to material decomposition.
It is therefore the object of the present invention to provide an injection moulding machine for producing qualitatively impeccable, wear resistant and durable parts, preferably miniature parts made of technical plastics, such parts being produced with as little waste as possible.
Miniature parts such as can be manufactured with the injection moulding machine according to the present invention, normally have a weight of between 0.001 grammes to 5 grammes. However, it is to be understood that parts with a weight of up to 50 grammes or more can be produced.
This task is inventively solved with an injection moulding machine with the characteristics of claim 1, and in particular with a machine in which the retention time of the plasticized, compressed hot plastics material in the collection chamber (also called piston tab or screw tab) is lowered by a shortening of the injection path of the melt. In particular, this shortening of the injection path is achieved by arranging the collection chamber immediately behind the outlet port of the nozzle tip.
In a first embodiment of the invention, the melt which is produced in a plasticizing cylinder is transported over a hot channel adapter into a nozzle body and is collected in a collection chamber within the nozzle body. The ready melt in this collection chamber is ejected with the aid of a piston and is pressed via injection channels into the cavities. In this embodiment, the ejection piston has an axial core (piston internal feed channel) having a lateral inlet port through which the melt is fed into the collection chamber, and also a ball valve at its tip, which closes off the collection chamber during ejection. The outlet port of the injection channels are closed or opened by means of a needle closure device.
In another embodiment of the invention, the melt is produced directly behind the hollow cylinder type ejection plug and is transported axially through this hollow cylindrical plug into the collection chamber (plug tab). Again, the collection chamber in the nozzle body can be closed off towards the feed or transportation channel of the hollow cylindrical plug, and the outlet ports of the nozzle tip can be sealed of with a needle shut-off device.
In a further embodiment of the invention, the melt is not fed through the piston itself, but is directly fed into the collection chamber. A sliding arrangement which is coupled to the needle closure device allows the collection chamber to be automatically closed off from the feed channel.
The advantages of the inventive injection moulding machine are immediately obvious for the man skilled in the art. Thus, the shortening of the injection path permits a more controlled increase in pressure in the cavity and enables an exactly dosed filling of the cavity with hot melt, which leads to injection moulded parts with little production tolerance (deviation from permissible value). Of course, with such a machine the cavities can be filled rapidly requiring only a low machine filling pressure. Energy savings and little wear and tear are further advantages of this shortened injection path.